This patent specification relates to optical amplifiers. More specifically, it relates to a semiconductor optical amplifier that amplifies an optical signal using energy from vertical cavity surface emitting lasers (VCSELs).
As the world""s need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are capable of very high-bandwidth operation.
Optical amplifiers are important components of optical communications links. In general, the two primary types of optical amplifiers are optical fiber based amplifiers, such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers, and semiconductor optical amplifiers (SOAs). EDFAs are widely used in line amplifiers and other applications requiring high output power, high data rates, and low noise. However, EDFAs are quite bulky, having a typical fiber length of about 30 feet, and require the presence of a separate pumping laser to operate. Accordingly, EDFAs are difficult to incorporate into confined spaces and are not amenable to circuit-board-level or chip-level integration.
SOAs, on the other hand, are small in size and conveniently integrated into small devices. However, conventional SOAs generally suffer from pattern-dependent gain fluctuations, which causes crosstalk in multiple-channel optical signals such as those present in wavelength division multiplexed (WDM) networks and dense WDM (DWDM) networks. Amplified spontaneous emission (ASE) noise is another primary troublesome noise source in conventional SOAs. ASE noise arises from random, spontaneous energy state drops in a small fraction of the excited carriers of the gain medium. Efforts continue toward reducing crosstalk effects and ASE noise in SOAs to increase their usefulness in WDM and DWDM networks, and for other applications.
WO 01/28049 (hereinafter the ""049 reference) discusses a vertical lasing semiconductor optical amplifier (VLSOA) in which an optical signal travels in a longitudinal direction along an amplifying path, the amplifying path including a semiconductor gain medium, the semiconductor gain medium forming the active medium of a plurality of vertical cavity surface emitting lasers (VCSELs) oriented vertically with respect to the amplifying path. The VCSELs are operated above threshold so as to cause lasing action therein. As the optical signal propagates through the active region, it is amplified by a gain multiplier due to stimulated emission of additional photons. The gain multiplier is substantially constant, i.e., independent of the amplitude of the optical signal, because the laser radiation produced by the VCSELs acts as a ballast to prevent gain saturation.
However, the VLSOA set forth in the ""049 reference may experience performance problems due to non-uniformities in the lasing field of the VCSELs. In a non-uniform lasing field, the photon density of the active medium contains undesirable variations that cause the gain experienced by the optical signal to vary, often unpredictably, with time and space within the active medium. One source of such non-uniformity involves the presence of higher-order transverse modes in the vertical cavities, the photon density concentrating in differing spatial patterns depending on which higher-order transverse mode is present. Moreover, when higher-order transverse modes are present, they may be highly unstable. The particular transverse mode dominating at any given instant may vary chaotically with even the smallest variations in excitation current. Operation of the SOA device is compromised in terms of gain multiplier magnitude, gain multiplier stability, and/or saturable power performance.
Another source of non-uniformity in the lasing field arises from practical problems encountered in real-world device fabrication. The growth of xe2x80x9cperfectxe2x80x9d epitaxial layers being extremely difficult or impermissibly expensive to achieve, real-world devices will have some statistical population of local defects in the semiconductor layers such as crystal dislocations, pitting, voids, etc. Such defects in the epitaxial growth can be a point of lower electrical resistance than the surrounding epitaxial areas. The higher electrical current flowing through these points of lower electrical resistance can create xe2x80x9chot spotsxe2x80x9d which cause spatially non-uniform currents in the affected areas of the gain medium. The spatially non-uniform currents can adversely affect the lasing action of the VCSEL cavities and cause non-uniform photon densities, again resulting in non-uniform gain and compromised device performance. The area of lasing non-uniformity in the gain medium can extend substantially beyond the immediate region of the local crystal defect. Moreover, the electrical current being funneled through a xe2x80x9chot spotxe2x80x9d from the surrounding regions can grow to such a magnitude that overheating and device failure can result.
Accordingly, it would be desirable to provide a vertically lasing semiconductor optical amplifier that is operationally robust in terms of gain multiplier magnitude, gain multiplier temporal stability, and saturable power performance.
It would be further desirable to provide a vertically lasing semiconductor optical amplifier having increased tolerance to local defects that may occur in the epitaxial growth stages of device fabrication.
A semiconductor optical amplifier (SOA) apparatus and related methods are provided for amplifying an optical signal, the SOA comprising an integrated plurality of vertical cavity surface emitting lasers (VCSELs) having a common gain medium layer, the SOA further comprising a signal waveguide extending horizontally through the VCSELs near the gain medium layer such that the optical signal is amplified while propagating therethrough, wherein each VCSEL is configured and dimensioned to achieve smooth, single-transverse-mode lasing action for promoting spatially uniform and temporally stable gain of the optical signal as it propagates along the signal waveguide. Although integrated onto a common substrate, the VCSELs are functionally isolated from each other, each building up its own distinct lasing field responsive to a distinct electrical pump current therethrough. Each VCSEL is configured and dimensioned to suppress higher order or otherwise uneven lasing modes at nominal bias levels. When each VCSEL is achieving smooth, single-transverse-mode lasing action at its nominal bias levels, the current density in the gain medium is even and temporally stable, thereby resulting in spatially uniform and temporally stable amplification of the optical signal.
According to a preferred embodiment, neighboring VCSELs are functionally isolated from each other by separation zones formed by electrically isolating implants. Preferably, the SOA comprises several dozens to several hundreds of functionally isolated VCSELs positioned along the optical signal path, the gain medium of each VCSEL providing only a small portion of the overall signal gain. Advantageously, if a local defect arises during device fabrication that causes a xe2x80x9chot spotxe2x80x9d to occur or that otherwise causes uneven lasing to occur at nominal bias levels, the spatial and operational scope of that defect is limited to its particular VCSEL. Furthermore, because that VCSEL is associated with only a small percentage of the overall signal gain, it is more likely that there will be only minor implications for overall device performance due to that local defect. According to one preferred embodiment, the VCSEL containing the local defect is operated at a lower bias level sufficient for gain medium transparency. According to another preferred embodiment, the bias level of that VCSEL is set to the lesser of (a) a nominal bias level for that VCSEL, or (b) a uniformity threshold bias level above which multi-transverse-mode or otherwise uneven lasing action is observed to occur in that VCSEL.
In one preferred embodiment, the plurality of VCSELs are positioned in a longitudinal array extending from an input end to an output end of the SOA, the VCSELs lasing vertically relative to the optical signal path that extends from the input end to the output end. Each VCSEL comprises a vertical arrangement of material layers including a set of lower distributed Bragg reflector (DBR) layers, a lower cladding layer, an active layer coextensive with the SOA gain medium layer, a current confinement layer defining a current aperture, an upper cladding layer, and a set of upper DBR layers. The signal waveguide is defined vertically by the upper cladding layer, the gain medium layer, and the lower cladding layer. The signal waveguide is defined laterally by a lateral confinement ridge formed at least in part by the upper DBR layers and extending from the input end to the output end of the longitudinal array of VCSELs.
According to one preferred embodiment, the separation zones that separate neighboring VCSELs are formed by electrically isolating implants that extend through the upper DBR layers, the upper cladding layer, and the gain medium layer. To reduce optical signal attenuation in the separation zones, their length is kept to a minimum amount that is still sufficient to isolate the neighboring VCSELs, which in one preferred embodiment is between about 1 xcexcm and 2 xcexcm. In another preferred embodiment, the gain medium is disordered in the separation zones to further reduce attenuation of the optical signal. In still another preferred embodiment, the electrical resistivity profile of the separation zones is manipulated so as to allow a small amount of pump current into the gain medium layer, thereby providing a degree of transparency thereto and reducing optical signal losses in the separation zones.
According to a preferred embodiment, to ensure single transverse mode lasing at nominal bias levels, the horizontal dimensions of each VCSEL are kept below predetermined lengths (in the direction of signal propagation) and predetermined widths (perpendicular to the direction of signal propagation) appropriate for the material system used and the wavelengths of device operation. For example, in one preferred embodiment, the current aperture length and output aperture length are each kept below about 10 xcexcm, and the current aperture width and output aperture width are also each kept below about 10 xcexcm. Preferably, the output aperture and the current aperture are roughly the same size. However, a variety of aperture shapes, aspect ratios, and dimensions are within the scope of the preferred embodiments that provide smooth, single-transverse-mode lasing action at nominal bias levels.